Here I wish to explain how the so called noble metal gold dissolves in Aqua regia. Aqua regia is a mixture of 3 parts of concentrated HCl and one part of concentrated HNO3. Aqua regia is supposed to be the best and also the only solvent for gold.
The dissolution of the noble metal gold occurs by the formation of a square planar complex between gold and chloride ions. The name of the complex is Hydrogen tetrachloroaurate(III). The stoichiometry of the reaction is as under:
3AuCl3 + HNO3 +3HCl -----------> 3H[Au(Cl)4] + 2H2O + NO
Aqua regia also solublises other metals like silver, nickel, cobalt etc.